1. Field of the Invention
The present invention relates, generally, to a headliner for use within a passenger compartment of a motor vehicle and, more specifically, to a headliner having acoustically attenuating properties.
2. Description of the Related Art
Headliners for motor vehicles are well known and generally comprise multi-layer panel type assemblies that are mounted inside the passenger compartment and against the roof of the vehicle to provide an aesthetic covering for the sheet metal and roof support structure. Conventionally, headliners expose a fabric or smooth finished layer, which is built up on a fibrous or foam material, to the interior of the passenger compartment. This interior layer, also known in the art as a “Class A” type surface, is typically affixed to a backing layer of a relatively strong, reinforced paper or plastic material, which is fastened to the inside of the roof of the vehicle. The backing layer may also be a composite layer that includes a type of bulk material to provide some level of sound deadening.
Due to economic factors, vehicle and headliner manufacturers are constantly under pressure to reduce headliner weight. Lighter weight headliners can lower the cost of manufacturing by reducing the amount of materials used in headliner, and generally lower the operating expense of the vehicle by contributing toward improving the gas mileage. On the other hand, the benefits of lighter weight headliners must be balanced with a competing issue of strength. A stronger headliner makes it easier for accessories such as dome lights, visors, mirrors and the like, to be installed directly to the headliner.
In addition to the economic issues mentioned above, the driving public places considerable importance on vehicle noise reduction. Since a large part of a vehicle's interior surface is covered by the headliner, efforts in noise reduction and sound control are strongly focused there. Previous manufacturers' attempts in noise reduction, as applied to headliner design, have generally forced a trade-off between desired weight reduction and the addition of heavy bulk materials that deaden or reduce the sound waves that pass through them.
To avoid the drawback of increased headliner weight by the addition of bulk material for sound absorption, some recent headliner designs have included resonating cavities formed in the upper layers. The resonating cavities create open areas that absorb sound while reducing the weight of the headliner. Properly designed resonating cavities can absorb and attenuate sound frequencies that reach them from any direction. In this way, undesirable external sound frequencies are absorbed as they enter the passenger compartment through the roof area, as well as those from other sources that are ambient in the vehicle interior. The physical nature of a resonating cavity is also such that it may be tuned by its particular size to resonate at, and thereby absorb, specific frequency ranges. Certain headliner designs have even incorporated a very efficient and frequency specific type of resonating cavity known as a Helmholtz resonator.
The typical approach to introducing resonating cavities into a headliner structure requires the insertion of additional layers or sheets of thin but rigid material into the middle layers of the headliner. These sheets are preformed to provide some type of undulating surface that creates the cavities on one side while other bulk material is packed or otherwise compressed about the other side as the headliner is formed. These special rigid sheets must be specifically manufactured to relatively close tolerances and then must be cut and sized to fit within the mold that produces the headliner. While the addition of the rigid sheets helps provide some inherent strength, they add to the cost of the headliner, both in materials and construction expense. Additionally, the bulk material that is cooperatively used with the rigid sheets is generally a loose fibrous material that may be packed and compressed against the undulations of the rigid sheets. The nature of the loose bulk material provides no physical strength itself and adds to the manufacturing complexity of the headliner as it must be held in its compressed state against the rigid sheet by another securing layer over top.
Thus, while the introduction of resonating cavities within the headliner has helped overcome some of the previous trade-offs between good sound absorbing qualities verses bulk headliner weight, current manufacturing techniques and the materials used in the construction of headliners having resonating cavities remain relatively costly. Therefore, there remains a need in the art for a lightweight, yet strong, headliner having easily formed resonating cavities that can be inexpensively assembled.